Optical absorption in metals is a fundamental loss process, which is difficult to avoid. When metals are nanostructured, the loss is typically more significant as the surface area increases and the structural size becomes comparable to the penetration depth of optical fields. In particular, when surface plasmons—collective oscillations of electromagnetic fields and free electrons in metal surfaces—are excited, energy dissipation can become even greater.
The absorption in metals often limits the efficiency of metal-based nanoscale devices. For example, in optoelectronic devices, such as nanophotovoltaic devices, light absorption in metal electrodes does not lead to electricity and decreases the efficiency of the device. In light emitting diodes, surface plasmons can be exploited to concentrate light but strong metal absorption remains a major source of losses in plasmon-mediated devices.
The problem of this parasitic absorption becomes critical when ultra-high efficiency devices are desired. Moreover, if the device operates over a broad band as in solar cells, metal absorption needs to be controlled over the whole spectrum of interest and this poses challenges.
When high refractive index materials and metals are used in a device, inserting a thin low index material between them often reduces absorption loss in metals. Moreover, metals are typically opaque at visible frequencies of light and become transparent only at frequencies higher than their plasma frequency, which is typically in the ultraviolet region. This unique optical property of metals has its origin in their free electron gas. Below the plasma frequency, the free electrons follow the electromagnetic oscillation of light and prevent the light from penetrating into the metal.
Oppositely, above the plasma frequency, the light wave oscillation is faster than the electronic movement and light can propagate in the metal. In many optical and optoelectronic applications, the frequency of light is typically well below the plasma frequency of the metals involved and light can penetrate only a few tens of nanometers into the metals.
It is also known that nanostructured metal-dielectric composites can become transparent. However, in this case, it has not yet been clear if light takes paths in the dielectric materials or actually penetrates into the metal. Only in the latter case, it would be possible for light to go into the dielectric regions surrounded by the metal. This property would be extremely useful for certain applications such as photovoltaics where the nanostructured active materials can be surrounded by metal electrodes. However, in typical nanostructured photovoltaic devices, a large surface to volume ratio causes serious losses of electric charge carriers due to their surface recombination.